Spike threshold dynamics in spinal motoneurons during scratching and swimming

Abstract

During functional spinal neural network activity motoneurons receive intense synaptic input, and this could modulate the threshold for action potential generation, providing the ability to dynamically adjust the excitability and recruitment order for functional needs. In the present study we investigated the dynamics of action potential threshold during motor network activity. Intracellular recordings from spinal motoneurons in an ex vivo carapace-spinal cord preparation from adult turtles were performed during two distinct types of motor behaviour - fictive scratching and fictive swimming. We found that the threshold of the first spike in episodes of scratching and swimming was the lowest. The threshold potential depolarizes by about 10 mV within each burst of spikes generated during scratch and swim network activity and recovers between bursts to a slightly depolarized level. Depolarization of the threshold potential results in decreased excitability of motoneurons. Synaptic inputs do not modulate the threshold of the first action potential during episodes of scratching or of swimming. There is no correlation between changes in spike threshold and interspike intervals within bursts. Slow synaptic integration that results in a wave of membrane potential depolarization rather than fast synaptic events preceding each spike is the factor influencing the threshold potential within firing bursts during motor behaviours.

Neuronal Population Activity in Spinal Motor Circuits: Greater Than the Sum of Its Parts

“…The majority of the neurons spent most of the time below threshold, i.e., in the fluctuation-driven regime (right, Figure 2D ), which may be relevant for muscular control since this regime represents 90% of the force modulation in mice (Manuel and Heckman, 2011 ). Since the threshold has a dependence on the firing rate (Grigonis and Alaburda, 2017 ) the initial threshold, i.e., the threshold of the first spike in a trial, was used in this analysis. It should also be noted that the impact of intrinsic properties, such as the spike frequency adaptation, on the spiking dynamics would be difficult to assess during the these intense synaptic input.…”